This invention relates to the use of optics to produce a directed beam of radiation by grazing-incidence reflection. More particularly, a monocapillary optic is disclosed which is produced by creating a precisely shaped mold which has the desired figure of the final capillary optic""s internal bore. The mold most commonly takes the form of a precisely etched wire. The mold is used as a mandrel for the production of a capillary optic by placing it between two polished and generally flat plates composed of a relatively soft material, and applying pressure. The profile of the mold is imprinted into the surfaces of the flat plates and is thus replicated. The plates are disassembled, the mold removed, and the two plates are reassembled to form the final capillary optic. In some instances, a reflection enhancing film is applied before the final assembly step. More than one mold can be used in the process to create a polycapillary optic. The resultant optics can be used to produce either a collimated beam or a focused spot of photons, neutrons, or charged particles.
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The introduction of x-ray analysis has been one of the most significant developments in twentieth-century science and technology. The use of x-ray diffraction, spectroscopy, imaging, and other techniques has led to a profound increase in our knowledge in all scientific fields. The capabilities of x-ray analysis have expanded consistently with the availability of ever more powerful sources of radiation. The standard x-ray tube has seen a relatively gradual increase in performance over many decades. Notable improvements in x-ray tube technology include the introduction of rotating anode sources and microfocus tubes. Although many experiments are now performed using rotating anode sources that have significantly higher power capabilities than stationary anode tubes, these sources are quite expensive and can consume over ten kilowatts of input power. Recently, with the introduction of improved x-ray focusing optics, the ability to use small, low power microfocus x-ray sources to achieve x-ray beam intensities comparable to that achieved with rotating anode tubes has been demonstrated. It has been demonstrated that a microfocus source running at a few tens of watts input-power, in conjunction with focusing optics, can produce beams with brightness comparable to a multi-kilowatt rotating anode source. Such combined small sources and collection optics will greatly expand the capabilities of x-ray analysis equipment in small laboratories. The optimization of x-ray optics for these applications is of crucial importance for realizing the potential of these laboratory instruments.
The advent of synchrotron radiation sources over the past several decades has produced a revolution in x-ray science. Due to the extreme brightness of these sources, measurements that were not possible in the past have become routine. This brightness allows the use of microfocusing optics to create very small x-ray microbeams with greatly enhanced flux densities. These microbeams have allowed measurements of samples with unprecedented spatial resolution and small size. Improvements to these optics are highly desirable to allow the full potential of synchrotron sources to be achieved.
In addition to sources of X-rays, there has been a significant increase in the capabilities of neutron sources, many of which exist as user facilities in a manner similar to synchrotron radiation sources. Both reactor and spallation sources have been built with ever increasing neutron fluxes. For certain applications such as prompt-gamma activation analysis and neutron depth profiling, it is very desirable to produce a focused beam of thermal or cold neutrons. The use of small beams is also advantageous for neutron diffraction applications. There have been advances in neutron focusing optics over the past few years. Further improvements to these optics will have a large impact on the capabilities of these neutron facilities.
There have been dramatic developments in the field of x-ray optics. Many different types of optical elements have been introduced or improved for the manipulation of short wavelength photons. In the specific area of focusing optics, several different types of lenses and focusing mirrors have been produced. These optics include: zone plates, Bragg-Fresnel zone plates, multi-layer-focusing mirrors, grazing incidence mirrors, compound refractive lenses, and capillary optics. In addition to x-ray photons, these optical elements can be used for the focusing of neutrons and charged particle beams.
In the area of capillary optics, there are two basic typesxe2x80x94monocapillary optics and polycapillary optics. The invention we will be discussing here involves improvements to both types of these optics. Although we will concentrate mainly on their use for the manipulation of x-rays, we will also discuss their use for the focusing and collimation of visible and near visible light. In particular, the use of the optics with laser light in fiber-optic communications applications is potentially very important and practical.
Although there exist x-ray optics that use reflection, diffraction, or refraction for their operation, we are concerned specifically with reflective optics in this invention. It is well known that x-rays incident on a surface at sufficiently small angles of incidence will be reflected by total external reflection. The largest angle of incidence for reflection (critical angle) is determined by the refractive index of the material:
n=1xe2x88x92xcex4xe2x88x92ixcex2
Using Snell""s Law, we can derive this angle as:
xcex8c=(2xcex4)1/2 (assuming xcex2=0)
The theoretical value for xcex4 is:
xcex4=xc2xd(e2/mc2)(N0xcfx81/A)Zxcex2=2.70xc3x971010(Z/A)xcfx81xcex2
The angles are quite small since the refractive index for x-rays is very close to unity for all materials. For example, the critical angle for borosilicate glass at xcex=1xc3x85 is less than 3 milliradians. For achieving the highest critical angles, high-density materials such as gold or platinum are desirable.
In the case of neutrons, grazing incidence reflection can also be used for producing optical devices. The critical angle for reflection of neutrons is:
xcex8c=xcex(Nb/xcfx80)1/2
Where b is the coherent scattering length and N is the number of nuclei per cm3. The best natural material for achieving the highest critical angle for neutron reflection is nickel. The isotope Ni-58 is especially good, having a critical angle of approximately 2.1 milliradians/xc3x85.
In addition to single-layer reflecting materials, multilayer coatings can be produced which rely on Bragg reflection to achieve high reflectivity. These layers are composed of a high-Z material that exhibits large coherent scattering for the radiation being reflected, with alternating low-Z layers that function as spacers. In the case of x-rays, the most common high-Z materials are tungsten or molybdenum, while the low-Z spacer is usually silicon, carbon, or beryllium. In the case of neutron reflection, these multilayer coatings are often referred to as xe2x80x9csupermirrors.xe2x80x9d Neutron supermirrors differ from standard x-ray multilayers in that the d-spacing of the layers is not constant, but increases for the layers towards the surface of the mirror. Supermirror structures are generally composed of layers of nickel or a nickel alloy, with spacer layers of titanium.
Reflective x-ray optics can be classified into several different categories. One class of optics uses grazing reflection from extended mirror surfaces. The most common mirror arrangements have an ellipsoidal or toroidal surface figure for two-dimensional focusing of radiation. Another common geometry uses two spherical mirrors oriented sequentially in the vertical and horizontal planes, in an arrangement known as a Kirkpatrick-Baez configuration. An absolute requirement for all reflective x-ray optics is the need to have exceedingly smooth reflecting surfaces due to the small wavelength of the radiation. In general, the surface roughness should be better than 1 nanometer rms.
A different approach for reflective x-ray optics uses the ability of fine glass capillaries to act as reflective guide tubes for x-rays, in a similar manner to fiber optics. Several different configurations of these capillary optics exist. One type of optic, sometimes known as a xe2x80x9cKhomakhov Lensxe2x80x9d, uses a number of discreet curved glass-capillary tubes that are precisely mounted in a frame that independently holds each capillary""s curved position along the device. In some optics, each carefully positioned capillary fiber is actually a bundle of many much smaller capillary tubes. X-rays are guided through each capillary by multiple reflections along the outer arc of the capillary tube""s interior surface. This mode of reflection is sometimes referred to as a xe2x80x9cWhispering Galleryxe2x80x9d. With such optics, the divergent radiation from an x-ray tube""s focal spot can be redirected into either a collimated beam, or condensed back to a small spot a significant distance from the source. This type of optic is not capable of producing spots smaller than approximately 500 microns; the diameter of each fiber. This type of optic is sometimes referred to as a xe2x80x9cmultifiber optic.xe2x80x9d
A different type of multicapillary optic uses a single multicapillary bundle that is drawn at elevated temperature so as to have a taper on one or both ends. This type of optic is sometimes referred to as a xe2x80x9cmonolithic optic.xe2x80x9d Unlike the multifiber optic, the individual glass channels do not have a constant diameter along their length. This type of optic functions in a similar manner to the multifiber type of optic, but has certain properties that are advantageous in some applications. The smallest spot sizes achievable with this type of optic are approximately 20 microns or larger, while quasi-parallel beam sizes are generally several mm in diameter. Both types of multicapillary optics possess the very attractive ability to collect a large solid angle of radiation emitted from the source. Their main disadvantage is their relatively high cost, and their inability to form very small focused or collimated beams. In addition, the reflecting surfaces are composed of bare glass, which has a relatively small critical angle. This causes the optics to cease to transmit efficiently at higher photon energies. Both the multifiber and the monolithic optics can also be used as bending optics by curving the capillary bundle. It is possible for a single optic to have a bending section and a focusing section.
Both the monolithic and the multifiber optics are manufactured and sold by X-ray Optical Systems which is located in Albany, N.Y. This company holds a number of patents regarding this technology.
In addition to the previously described capillary optics having multiple channels, a single tapered-capillary may be used effectively to collimate or focus radiation. This tapered monocapillary optic has achieved the smallest spot sizes of any type of x-ray optical device. Their most dramatic use has been with synchrotron radiation where the almost parallel input beam can be condensed to sizes well below 1 micron (0.05 microns has been achieved). It is important to note that tapered monocapillary optics can function in two different focusing modes. In one case, the capillary acts as a true focusing lens, with each photon undergoing a single bounce. In this case, a focal spot is produced some distance beyond the capillary exit. In a different type of capillary optic, photons undergo multiple bounces on their way to the exit and the smallest beam diameter is found directly at the exit aperture. This type of optic is often referred to as a capillary condenser, to draw the distinction from a true lens. The smallest beams have been produced by the condenser type optics, although the small working distance from the exit aperture can be a disadvantage.
A glass monocapillary optic is produced by heating and drawing a length of glass capillary tubing to a smaller diameter. There has been significant progress in forming glass capillaries with paraboloidal or ellipsoidal shapes, but the slope errors and straightness of the capillary shapes have been difficult to reliably reproduce. It has also not been feasible to coat the inside of these very small capillaries with a different material to enhance the reflectivity at higher energies or reflection angles. In addition to a low angle for total external reflection, glass has the disadvantage of transmitting a significant amount of the radiation that is not reflected by the capillary bore due to its low density and atomic number. This radiation continues to propagate through the glass material and passes out through the end of the optic. These photons produce what is known as the xe2x80x9chalo effectxe2x80x9d in both monocapillary and polycapillary type optics, and has the effect of increasing the effective size of focused beam.
In addition to the glass monocapillary optics produced by the drawing method, a different technique has been devised to produce metal reflective optics by a replicating process on a removable mandrel using electroforming. The optics produced by these replication methods fall into a category somewhere between standard reflecting optics and true capillary optics, due to their larger bore size. The smallest dimensions of their bores are generally near 0.5-1 mm. These replicated optics are produced by Reflex s.r.o in the Czech Republic.
U.S. Pat. No. 5,772,903 (1998) entitled TAPERED CAPILLARY OPTICS describes a different technique to produce tapered monocapillary optics that have significant advantages over the tapered glass capillaries. That patent describes a method to produce a capillary structure having a well-controlled taper profile, a high degree of straightness, an extremely smooth internal reflecting surface, and wide latitude in the selection of materials for the internal bore. The extremely small dimensions ( less than 1 micron) achievable by the methods delineated in that patent are unique for non-glass capillaries.
U.S. Pat. No. 6,126,844 (2000) entitled TAPERED MONOCAPILLARY-OPTICS FOR POINT SOURCE APPLICATIONS describes certain improvements to the process for producing monocapillary optics described in U.S. Pat. No. 5,772,903. This patent is especially directed at applications where the radiation originates from a point-like source. Both paraboloidal, and full ellipsoidal capillary shapes are disclosed.
In both of the above referenced patents, a precisely tapered wire is first produced by the precise differential etching of a starting wire material. The shape of this etched wire is most commonly paraboloidal or ellipsoidal. The wire is then coated with a layer chosen to maximize the final optic""s reflectivity. A thicker reinforcing layer is applied after the reflective coating. This coated wire is then mounted to a substrate, and the wire is removed to leave the hollow bore of the final optic. The wire removal is most often achieved by a chemical etching process that is facilitated by the cutting of a number of narrow grooves perpendicular to the optic axis. Since the wire functions as a mandrel for subsequent forming operations, we refer to it as the xe2x80x9cmandrel wirexe2x80x9d.
A pending U.S. patent, BUNDLED MONOCAPILLARY OPTICS Ser. No. 09/503776 filed Feb. 14, 2000 now U.S. Pat. No. 6,415,086, describes techniques to extend the technology of the above patents into the realm of polycapillary optics. In this patent, methods to create large numbers of wires in a batch operation are described. These wires are coated with a reflecting film, bundled together into an array, and fused together into a rigid monolithic structure. The wires are then removed, in a similar manner to the monocapillary optic. The advantages of these metal optics compared to glass polycapillaries are the same as those pertaining to monocapillary optics.
The invention we will be disclosing here involves an alternate process for forming capillary optics from precisely formed mandrel wires. Instead of the vacuum deposition and electroplating operations disclosed in the previous patents, a purely mechanical process in disclosed for the replication of a wire. The advantage of this process over the previous methods is a much simpler process having fewer processing steps. The resultant optics are free of any etching process generally required in the other methods, which permits a higher degree of cleanliness on the internal surface. This is especially important for low energy x-ray performance. The absence of a chemical etching process also expands on the number of possible reflecting coatings since there are no issues of chemical compatibility to be concerned with.
A mold is formed into the shape of a capillary optic""s desired internal bore. This shape is most commonly paraboloidal or ellipsoidal. This shape is generally, but not necessarily, created by carefully controlling the withdrawal speed of an initially uniform wire from an etchant bath. In the case of a complete ellipsoidal capillary, the etching operation is performed twice in opposite directions on adjacent wire segments. The etched wire undergoes a subsequent operation to create an extremely smooth surface if the as-etched surface is not smooth enough. The wire is then placed between two rigid plates. In the most common embodiment, the two plates are of identical composition and are relatively soft materials, with the initial surfaces being substantially flat and highly polished. This plate-wire-plate sandwich is then mechanically pressed. This results in the wire profile being imprinted in relief into the plate surfaces. The plates are disassembled, and the wire is removed. These surfaces can then be coated with a layer of material that is selected to maximize the reflectivity of the radiation that will be focused. In some cases, this reflection enhancing coating is deposited before the pressing step. This reflective surface may be a single layer for wideband reflectivity, or a multilayer coating for optimizing the reflectivity in a narrower wavelength interval. The two plates are then carefully aligned, and attached together to form the final capillary optic. In some devices, more than one wire is incorporated in the pressing step, which creates a polycapillary optic. The capillary optic is used for efficiently collecting and redirecting radiation from a source which could be the anode of an x-ray tube, a plasma source, the fluorescent radiation from an electron microprobe, a synchrotron radiation source, fiber optic cables, lasers, neutrons sources, or other radiation sources.